I am also of the opinion that the army - at least half of it, if not more - will attack their own people. This is casually happening all over the world, for thousands of years. Right now Syrian army is killing Syrians; Egyptian army is preparing to kill Egyptians; every civil war is like that, and we already saw quite a few in this century. The hope that the US Army will all desert has no foundation in fact. I can accept that, say, 10% to 20% of soldiers will disappear over time, if they can do so. The rest will not be given a chance. One man cannot easily escape from guarded barracks; if captured, he will be shot in front of other soldiers as a lesson. Mass desertion is possible only if it is truly mass desertion, when large groups of troops - guards and officers and all - quietly walk out without a single shot and nobody tries to hunt them down.
you will need not only armed citizens, but armed citizens who will commit to fight, watch their families massacred in reprisal and keep on fighting against a vastly stronger foe.
That is a very difficult choice to make when the alternative is to accept slavery and submit but stay alive. People can choose fight over slavery, and they did that on many occasions in recorded history. But that requires very serious commitment, the willingness to die for one's beliefs. People also on far more numerous occasions submitted to the force, especially if the new oppressor was not too harsh. (As an example, a lot of Guerilla movement on Germany's Eastern front was instigated by cruelty of Nazi troops. They were worse than the communists! That's what forced locals to resist, not just the propaganda of the old regime.)
Given that the majority is ready and willing to drop their pants at the TSA checkpoint, I do not really expect a sufficient number of people to seriously oppose the government. But low numbers of opposition, as well as uncoordinated actions, will allow the government to label them as "militants" or "domestic terrorists", and the soldiers will be more willing to eradicate them, in the Branch Davidians' style. The soldiers will be, generally, taking the side of the majority. We already know who the majority is.
On the other hand, insurgents are usually a minority. Most colonists were loyal to King George III, but the rebels were more passionate in their rebellion then the loyalists were in their opposition to rebellion, had very talented leaders, and eventually help from France. Same holds with the revolt against the British Mandate in Israel. It was carried out by a small, but well-organized and highly motivated group of “extremists”. Being a minority has the advantage of tight organization, and intimate bonding between the band of brothers as one fights against severely daunting odds. Being a majority means that you have a large organization that is permeable through espionage, raids, inattention and defections. So often the advantage is with the minority.